Direct current servo mechanism

ABSTRACT

The control characteristics of potentiometers driven by the input and output shafts are compared and imbalance therebetween utilized to drive the output shaft motor in a direction to restore the balance. To avoid ambiguity in operation the potentiometers have an effective control characteristic range extending beyond 360* of shaft rotation to provide an overlap between the beginning and end of their ranges, which overlap is at least as great as the operational angular lag between the shafts. Means are provided for switching both potentiometers simultaneously from near the end of their ranges back to near the beginning of their ranges with such switchover occurring in the area of the overlap.

451 May 1, 1973 [54] DIRECT CURRENT SERVO MECHANISM George M. Holley, Jr., 510 Sheldon Road, Grosse Pointe Shores, Mich. 48236 221 Filed: Mar.30, 1972 211 Appl.No.:239,453

[76] Inventor:

[56] References Cited UNITED STATES PATENTS Primary Examiner-Bamard A. Gilheany Assistant ExaminerW. E. Duncanson, Jr. Attorney-Burton and Parker [57] ABSTRACT The control characteristics of potentiometers driven by the input and output shafts are compared and in balance therebetween utilized to drive the output shaft motor in a direction to restore the balance. To avoid ambiguity in operation the potentiometers have an effective control characteristic range extending beyond 360 of shaft rotation to provide an overlap between the beginning and end of their ranges, which overlap is at least as great as the operational angular lag between the shafts; Means are provided for switching both potentiometers simultaneously from near the end of their ranges back to near the beginning of their ranges with such switchover occurring in the area of the overlap.

12 Claims, 6 Drawing Figures 2,417,0l5 3/1947 Razelt ..318/664 3,254,283 5/1966 Hunt ..3l8/663 3,668,503 6/1972 Lindner ..3l8/665 PATENTEUHAY 11913 3.731.171

AMP Z4 DIRECT CURRENT SERVO MECHANISM BACKGROUND OF THE INVENTION This invention relates to a direct current servo mechanism which is effective to cause an output shaft to accurately track an input shaft without ambiguity. The system would have application wherever the cost or other considerations might dictate against using a conventional alternating current servo mechanism. In particular I have conceived of this system for use in conjunction with an automatic position plotter for pleasure boats and the like and in this regard the system would have particular, though not exclusive utility, in connection with the plotter disclosed in my US. Pat. Application Ser. No. l99,24l,filed Nov. 16, 1971.

To keep manufacturing cost down, an important consideration in contemplating equipment for the pleasure boat market, readily available inexpensive components should be used whenever possible. An ideal shaft position sensor would be a conventional rotary potentiometer, but such potentiometers have a deadband or segment of instability of about 30. This is normally blocked out by the use of a stop limiting rotation of the sweep contact to about 330. The second problem, even assuming some solution to the deadband feature of potentiometers, is that at the point in potentiometer rotation where the range of control characteristic repeats, or starts over, ambiguity is apt to occur in rotation of the output shaft of the servo as it tracks the input shaft. For example, assuming an input shaft and a reversibly motor driven output shaft with a potentiometer connected to each shaft, and with means for comparing the control characteristics of the potentiometers and driving the output shaft motor in accordance with the relative imbalance therebetween, when the input shaft potentiometer turns a full 360 and its range begins to repeat, its control characteristic will suddenly drop (or suddenly increase, depending on the direction of potentiometer rotation). If up to this point in its rotation the control value of the input shaft potentiometer had been slightly greater than the control value of the output shaft potentiometer, thereby producing an imbalance which was causing the output shaft motor to track the rotation of the input shaft, the sudden drop in the value of the input shaft potentiometer will create a reverse imbalance in values and the output shaft motor will reverse and start driving the output shaft in the opposite direction from the input shaft to cause equalization of values. While such ambiguity may be avoided by never turning the input shaft beyond 360, such a limitation would be a serious objection in a servo mechanism where the input shaft may often or even occasionally rotate more than 360. Where continuous unambiguous tracking over more than 360 is required, this limitation has ruled out, so far as I am aware, the use of potentiometers as a shaft position sensor in a servo mechanism.

One object of my invention is the provision of a servo mechanism which operates on direct current.

Another object is the provision of a servo mechanism capable of accurately tracking over more than 360 of input shaft rotation and wherein readily commercially available potentiometers may be used as the shaft position sensors or as the means for providing a control characteristic dependent upon shaft position, without the deadband inherent in such potentiometers introducing instability into the servo system.

Another object of the invention is to provide a direct current servo mechanism utilizing potentiometers as the means for providing the control characteristic for the input and output shafts and wherein no ambiguity in tracking arises as the control characteristic cyclically repeats when the potentiometers exceed 360 of rotation.

These and other features will become more apparent as the description proceeds.

SUMMARY OF THE INVENTION In general my servo mechanism includes an input shaft and a reversible DC motor driven output shaft. Connected to each shaft is a potentiometer which has a control ((1) It is appreciated that a potentiometer is fundamentally a voltage divider and that the control characteristic sensed by the transistors of the differential amplifier herein disclosed is either voltage or current.) range which extends beyond 360 of the shaft rotation such that opposite ends of the range in effect overlap the 360 interval of the input and output shafts rotation. Means are provided for comparing the values of the potentiometers and driving the output shaft motor in a direction to equalize them. And other means are provided for simultaneously switching the input and output shaft potentiometers from near the end of their ranges to near the beginning of their ranges when the 360 interval of one of the shafts is reached. The deadband of the potentiometers is avoided either by using a pair of potentiometers in 180 displaced phase relation and switching back and forth between them at 180 intervals so that neither is relied upon during their deadband segments, or a potentiometer is connected by a gear reducer to the input shaft and another to the output shaft with a pair of sweep contacts for wiping the resistance segment and switching between the sweep contacts as one nears the end of the resistance segment.

DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 disclose one form of my servo mechanism utilized in conjunction with an automatic radio direction finder and where the output side of the servo controls the position of a pointer .or the like indicating the angular position of an antenna in the A.D.F.

FIG. 4 shows the relationships of input and output shaft rotation to control values of the potentiometer assemblies associated with the shafts;

FIG-5 is another form of my servo mechanism; and

FIG. 6 shows the relationship of input and output shaft rotation to the control values of the potentiometers associated'with the shafts.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS In the drawings the invention is shown in connection with an automatic radio direction finder and would -have application, among other things, where it is tenna is kept continuously oriented toward a distant radio signal transmitter. Representative systems are shown in my U.S. Pat. Nos. 3,419,866 and 3,623,102 and need not be further described here. The antenna position indicator is shown at 26 in FIG. 3 sweeping a bearing card or the like 28.

Fundamentally, the object is to cause output shaft 30, upon which pointer 26 is mounted, to track accurately the rotation of shaft 12 upon which the antenna is mounted, and to accomplish this using only direct current. DC. motor 14 is connected to what may be termed an input shaft 32 which carries a cam 35 which at every 180 of shaft rotation operates a pair of two position switches 34 and 36 (only one of which is represented for clarity in FIG. I). Shaft 32 carries a pinion gear 38 which meshes with ring gears 40 and 42 in a 1:1 ratio with the ring gears connecting a pair of rotary potentiometers 44 and 46 in 180 out-of-phase relation. If desired the potentiometers may be ganged and driven directly by shaft 32. Output shaft 30 is driven by DC. motor 48. A pinion gear 50 mounted on shaft 30 meshes with ring gears 52 and 54 in a 1:1 ratio which drive, respectively, potentiometers 44 and 46 in 180 out-of-phase relation. Potentiometers 44 and 46 may be ganged and driven directly by shaft 30.

The pair of potentiometers 44 and 46 comprise an input shaft potentiometer assembly, and the pair of potentiometers 44' and 46 comprise an output shaft potentiometer assembly. Each potentiometer assembly provides a rising or falling control value or characteristic over a range in excess of 360 of rotation of their respective output or input shafts as will become apparent hereinafter. Each such assembly is responsive to the rotation of its shaft to produce a control characteristic, specifically a voltage, which rises and falls in magnitude as a function of shaft position and in accordance with the direction of shaft rotation and which cyclically repeats itself at substantially corresponding points of shaft rotation. I

In FIG. 2, means are shown for effectively comparing the control characteristics of the input and output shafts and driving motor 48 in one direction or the other dependent upon the relative magnitudes of the control characteristics of the potentiometer assemblies to cause shaft 30 to track the rotation of the input shaft 32 and in so doing the position of the antenna 10. A pair of transistors 56 and 58 are connected in conventional manner to resistors 60, 62 and 64 to form a differential amplifier which drives a conventional solid state logic 66 which in turn drives motor 48. The bases of transistors 56 and 58 are connected respectively to switches 34 and 36 under the control of cam 35. Switches 34 and 36 determine which potentiometer of each pair is to be connected in the base circuit of the transistors.

The potentiometers of each potentiometer assembly are connected by the pinion and ring gears for simultaneous rotation in 180 out-of-phaserelation. Each potentiometer is an essentially readily commercially available component. Such potentiometers usually are provided with a stop limiting rotation to approximately 330 to avoid a deadband or unstable range. In my design each such potentiometer has the stop removed so that it can rotate a full 360.

Referring to FIG. 4, I show schematically the relationships of potentiometer control values of the input and output shaft potentiometer assemblies to shaft positions. Along the X axis shaft positions are plotted for somewhat more than two complete revolutions of the input and output shafts. Along the Y axis the control values in terms of voltage are plotted in say one volt increments. As each shaft rotates clockwise, and assuming that switches 34 and 36 have connected potentiometers 44 and 44 to the base circuit of transistors 56 and 58 in FIG. 2, the voltage in such increases until input shaft reaches 180 of rotation, and at this point switches 34 and 36 will disconnect potentiometers 44 and 44 from the base circuits and connect potentiometers 46 and 46' to the base circuits of the transistors, and as the shafts continue to turn clockwise the voltage values of the potentiometer assemblies will likewise continue to rise. At 360 of input shaft rotation, switches 34 and 36 will switch from the control characteristics produced by potentiometers 46 and 46' back to the characteristics produced by 44 and 44. Such cyclic repetion of the control characteristics will continue during continued rotation of the input and output shafts. If the shafts rotate counterclockwise the reverse will occur, i.e., the values will decline. The important feature of this arrangement is evident from a consideration of the effect on the circuitry shown in FIG. 2.

In FIG. 2, when a clockwise rotation of input shaft 32 causes potentiometer 44 to increase the voltage in the base circuit of transistor 56 above that in the base circuit of transistor 58 created by potentiometer 44, the arrangement of the logic system 66 is such that motor 48 is driven in a direction that will cause shaft 30 to turn potentiometer 44 to increase the voltage in the base circuit of transistor 58 to create a balance. Therefore, as the voltage of input shaft potentiometer assembly increases, the output shaft will turn clockwise, and should the relative voltage values reverse, the output shaft will turn in the opposite direction.

It will be apparent from a study of FIG. 4, that as long as the potentiometers 44 and 44 are connected to their respective base circuits, or alternatively potentiometers 46 and 46', that the output shaft will track the input shaft in either direction of input shaft rotation. By virtue of having the potentiometers 44 and 46, and 44 and 46', connected in I out-of-phase relation and then switching between them simultaneously in the area where their ranges overlap, no ambiguity in the tracking will occur. If one assumes that 300 of potentiometer rotation may be reliably utilized without instability arising from the deadband area, this allows an overlap of between the pairs of potentiometers of each assembly at each switch-over point (represented by the vertically dashed lines in FIG. 4). As this 120 area of overlap yields 60 before or after the switchover, the output shaft may lag behind the input shaft by 60 in either direction of input shaft rota- .tion without ambiguity in tracking. If a greater number of potentiometers in each potentiometer assembly, or more than 300 of potentiometer rotation were to be utilized, an even greater lag between input and output shafts could be tolerated. However, 60 will probably,

I in most instances, be sufficient once the input and output. shafts are initially synchronized.

To illustrate the switch-over feature in FIG. 4, assume that both potentiometers 44 and 44 are at equal voltages represented by points S and S. Then assume input shaft 32 is suddenly rotated clockwise to a position of 190 and a value of T, and motor 48 begins to drive the output shaft clockwise within the permissible 60 of allowable lag behind shaft 32. When the input shaft reaches 180 the switches 34 36 will effect a simultaneous switchover from potentiometers 44 44' to 46 46 and the voltage will jump from approximately 3 volts to 4 volts in the input potentiometer as sembly. However, simultaneously the voltage will jump one volt in the output potentiometer assembly to T so that the relative relationship in control characteristic will be preserved, and if the input shaft stops at T, the output shaft will stop at T". The same would be true if reverse rotation were effected by the input shaft. Thus accurate tracking may be effected utilizing direct current and no ambiguity in rotation of the output shaft will occur.

In considering FIG. 4, the voltage values of the input potentiometer assembly and the voltage values of the output potentiometer assembly while passing through two discrete overlapping steps may be thought of as increasing from a minimum of say one volt at shaft positions of 300 to a maximum of 6.5 volts 480 later during clockwise rotation of the shafts. This range cyclically repeats itself as shown in FIG. 4, with an overlap of 120 at each 360 interval ofshaft rotation. If this range of from one to 6.5 volts is considered as the range of the control characteristic, then when the switches 34 and 36 operate at the 360 intervals of input shaft rotation, the control characteristic is shifted from near the end of its range back to near the beginning of its range. While at the 180 intervals of input shaft rotation there is also a switchover, this may be considered as occurring within the range of the control characteristic rather than as a switch from near the end back to near the beginning of the range. By providing this overlap at the beginning and end of the range of the control characteristic and switching over within the overlap, any ambiguity of operation of the output shaft motor 48 is avoided as long as the lag between input and output shafts does not exceed the overlap. By using ,two potentiometers in each potentiometer assembly, with the potentiometers arranged 180 out-of-phase, and then relying on their voltage values for only 300 of the rotation of each, and simultaneously switching from the range of one potentiometer in each pair to the other potentiometer in each pair at each 180 of input shaft rotation, any instability arising from their deadband region may be avoided.

While I have shown a differential amplifier as the means for comparing the control characteristics of the input and output shafts, other comparing means will occur to those skilled in the art; Also, instead of effecting the switching from one range to another range by sensing rotation of the input shaft, the system may employ a voltage sensitive relay in the base circuit of the input transistor 56 which will effect a switchover to the other range. And those skilled in the art may elect to sense either output shaft rotation or voltage in that circuit to effect the switchover. All such expedients are clearly within the scope of this disclosure,

In FIGS. 5 and 6, I have shown a modified form of my servo system which differs from that described above in the following respects. Instead of two potentiometers in each potentiometer assembly, only one potentiometer for the input shaft 32 and one potentiometer for the ouput shaft 30 are utilized, and such are shown respectively at and 70. These potentiometers are so designed or connected to their respective input and output shafts that the shafts can rotate 480, for example, before the potentiometer cycle repeats. This may be accomplished most simply by connecting the potentiometers totheir respective input and output shafts by a gear reducing train (not shown) which will allow their shafts to rotate 480 while the potentiometers sweep, for example, 330. In FIG. 6 I have represented the relationship of the input and output shaft potentiometer voltage ranges for somewhat in excess of two full revolutions of the input and output shafts. Each potentiometer has two sweep contacts shown in FIG. 5 at 72 and 74 for the input potentiometer and 72 and 74' for the output potentiometer. The switches 34 and 36 switch the bases of transistors 56 and 58 between such sweep contacts as described in connection with FIG. 2.

In operation of the FIG. 5 arrangement, and having reference to FIG. 6, assume that the input and output shafts are stationary and that the voltage values (con trol characteristics) in the base circuits of the transistors 56 and 58 are therefore equal, as at S and S in FIG. 6. Then assume the input shaft suddenly rotates clockwise to just beyond its 360 position, viz., just to the right of the dashed vertical line in FIG. 6. As the input shaft passes the 360 point, the switches 34 36, which in this embodiment are actuated only every 360 rather than every 180 as in the previously described embodiment, cause a switchover of the control characteristic of each potentiometer from near the end of its voltage range to near the beginning of its range. In FIG. 5 thiswould involve switching from say sweep contacts 72 and 72 to contacts 74 and 74. At the time of this switchover the respective values of the input and output potentiometers might be represented as T and T in FIG. 6.

As soon as the switchover from the end of the voltage ranges of the potentiometers to the beginning of their ranges is effected, the new voltage values might be represented as shown at U for the input potentiometer and U for the output potentiometer, and it will be noted that both before and after the switchover the voltage of the input potentiometer exceeds that of the output potentiometer so that the motor 48 will continue to rotate in the same direction until, when the input shaft stops with a voltage value at V, the motor 48 will continue to operate until the output shaft potentiometer voltage is at V. Of course, counterclockwise rotation of the input shaft will cause a reversal in the voltage values and a switch back to the other end of the voltage ranges of the potentiometers in like fashion.

Of importance is that the beginning and end of the voltage ranges for the output shaft potentiometer (and also the input potentiometer for the same reaons heretofore mentioned in connection with FIGS. 2 and 4) overlap as shown in FIG. 6 and that the switchover from the end of the range to the beginning occurs simultaneously for both potentiometers and within this overlap. This avoids ambiguity in the operation of motor 48 as previously discussed.

It will now be apparent that with either the system of FIG. 2 or that of FIG. 5 there are means responsive to the rotation of the input shaft (in FIG. 2 the potentiometer assembly of potentiometers 44 and 46, and in FIG. 5 the potentiometer 70) for producing a control characteristic and similar means responsive to rotation of the output shaft for producing a control characteristic, and that such control characteristics rise and fall in magnitude as a function of shaft position and in accordance with the direction of shaft rotation and that they repeat themselves at substantially corresponding points of their shafts rotation. Further, it is apparent that the control characteristic for each shaft, but particularly the output shaft, extends beyond 360 of shaft rotation, and the amount it extends beyond 360 of shaft rotation, which has been termed the range overlap, is at least equal to the operational angular lag between the input and output shafts.

In the case of the FIG. 2 and 4 system this overlap of control characteristics produced by the pair of potentiometers of each potentiometer assembly is also at least as great as the operational angular lag between the input and output shafts.

Further it will be noted that means are provided, shown as a conventional differential amplifier and solid state logic system, for comparing the control characteristics of the input and output shafts and driving the output shaft motor 48 in one direction or the other in accordance with the relative magnitudes of the control characteristics. Finally, means are provided in the form of the switches 34 and 36 and the cam 35 for switching simultaneously the rising or falling control characteristic of the input and output shafts in the area of the overlap of the control characteristic of each from the end of the range of the control characteristic to the beginning of the range.

In the foregoing description the overlap of the control characteristic or voltage ranges at the 180 and/or 360 intervals is shown as the same for both the input potentiometer 44 46 or 70, and the output potentiometer 44' 46' or 70. In fact this is only a matter of simplifying design as the overlap for the input potentiometer may be slight, viz., only sufficient to insure proper switchover between voltage steps or from the end of its range to the beginning of its range. However, a greater overlap for the output potentiometer would normally be provided for the reasons mentioned above.

It is to be understood that the voltage values shown in FIGS. 4 and 6 are merely representative, and if desired to obtain greater drive in the logic circuit for motor 48, the voltage ranges may be from say 4 6 volts. Also, in FIG. 4 l have shown potentiometers 44 and 46 and again potentiometers 44' and 46' as being of different resistive values so that the voltage conof shaft rotation and which cyclically repeats during shaft rotation,

means for comparing the control characteristics of the shafts and driving the motor in one direction or the other dependent upon the relative magnitudes I of the control characteristics,

and means for switching simultaneously the rising or falling control characteristics of both shafts to the other end of their respective cycles when the control characteristic of one of the shafts approaches the end of its cycle.

2. The invention defined by claim 1 characterized in that said means for switching the control characteristics switches both to the other end of their cycles when the control characteristic of the input shaft approaches the end of its cycle.

3. The invention defined by claim 1 characterized in that said means for switching the control characteristics is responsive to rotation of the input shaft to a position where its control characteristic approaches the end of its cycle to switch the control characteristics to the other ends of their cycles.

4. The invention defined by claim 1 characterized in that said means for producing a control characteristic for each shaft produces a characteristic which extends beyond 360 of shaft rotation providing an overlap of the beginning and end of the characteristic which is angularly equal to at least the operational angular lag between the input and output shafts.

5. The invention defined by claim 4 characterized in that said means for switching the control characteristic switches both control characteristics at a point during the rotation of one of the shafts lying within said overlap.

6. The invention defined by claim 1 characterized in that the control characteristic for the other shaft extends beyond 360 of such shafts rotation providing an overlap of the beginning and end of the characteristic which is angularly equal to at least the operational angular lag between the shafts.

7. The invention defined by claim 6 characterized in that said one of the shafts is the input shaft and said other shaft is the output shaft.

8. The invention defined by claim 6 characterized in that there is an overlap of the beginning and end of the control characteristic for said one of the shafts.

9. The invention defined by claim 1 characterized in that said means for producing the control characteristic produces a characteristic for each shaft comprising a plurality of discrete overlapping steps, said means for switching the control characteristics switches the same when in the area of overlap of the control characteristic for said one shaft, and the angular length of the overlap of the steps for the other shaft being at least equal to the operational angular lag between the shafts.

10. A servo mechanism comprising:

a rotary input shaft and a rotary output shaft,

a motor connected to the output shaft to rotate it in either direction,

rotary potentiometer means connected to each shaft to be driven thereby and producing a control characteristic which extends beyond 360 of shaft rotation providing an overlap of the beginning and end of the characteristic,

11. The invention defined by claim 10 characterized in that each potentiometer means comprises a pair of potentiometers connected for simultaneous rotation out of phase.

12. The invention defined by claim 11 characterized in that said switching means comprises a pair of two position switches responsive to rotation of the input shaft and connected to the pairs of potentiometers to switch simultaneously from one potentiometer of each pair to the other every 180 of input shaft rotation. 

1. A servo mechanism comprising: a rotary input shaft and a rotary output shaft, a motor connected to the output shaft to rotate it in either direction, means responsive to the rotation of each shaft for producing a control characteristic for that shaft which rises and falls in magnitude as a function of shaft position and in accordance with the direction of shaft rotation and which cyclically repeats during shaft rotation, means for comparing the control characteristics of the shafts and driving the motor in one direction or the other dependent upon the relative magnitudes of the control characteristics, and means for switching simultaneously the rising or falling control characteristics of both shafts to the other end of their respective cycles when the control characteristic of one of the shafts approaches the end of its cycle.
 2. The invention defined by claim 1 characterized in that said means for switching the control characteristics switches both to the other end of their cycles when the control characteristic of the input shaft approaches the end of its cycle.
 3. The invention defined by claim 1 characterized in that said means for switching the control characteristics is responsive to rotation of the input shaft to a position where its control characteristic approaches the end of its cycle to switch the control characteristics to the other ends of their cycles.
 4. The invention defined by claim 1 characterized in that said means for producing a control characteristic for each shaft produces a characteristic which extends beyond 360* of shaft rotation providing an overlap of the beginning and end of the characteristic which is angularly equal to at least the operational angular lag between the input and output shafts.
 5. The invention defined by claim 4 characterized in that said means for switching the control characteristic switches both control characteristics at a point during the rotation of one of the shafts lying within said overlap.
 6. The invention defined by claim 1 characterized in that the control characteristic for the other shaft extends beyond 360* of such shaft''s rotation providing an overlap of the beginning and end of the characteristic which is angularly equal to at least the operational angular lag between the shafts.
 7. The invention defined by claim 6 characterized in that said one of the shafts is the input shaft and said other shaft is the output shaft.
 8. The invention defined by claim 6 characterized in that there is an overlap of the beginning and end of the control characteristic for said one of the shafts.
 9. The invention defined by claim 1 characterized in that said means for producing the control characteristic produces a characteristic for each shaft comprising a plurality of discrete overlapping steps, said means for switching the control characteristics switches the same when in the area of overlap of the control characteristic for said one shaft, and the angular length of the overlap of the steps for the other shaft being at least equal to the operational angular lag between the shafts.
 10. A servo mechanism comprising: a rotary input shaft and a rotary output shaft, a motor connected to the output shaft to rotate it in either direction, rotary potentiometer means connected to each shaft to be driven thereby and producing a control characteristic which extends beyond 360* of shaft rotation providing an overlap of the beginning and end of the characteristic, the rotary potentiometer means connected to one of the shafts having an overlap of the beginning and end of its control characteristic which is angularly equal to at least the operational angular lag between the input and output shafts, means for comparing the control characteristics of the potentiometer means and driving the motor in one direction or the other dependent upon the relative magnitudes thereof, and means for switching simultaneously the control characteristics of both shafts from one end of their cycles to the other in the area of their overlaps.
 11. The invention defined by claim 10 characterized in that each potentiometer means comprises a pair of potentiometers connected for simultaneous rotation 180* out of phase.
 12. The invention defined by claim 11 characterized in that said switching means comprises a pair of two position switches responsive to rotation of the input shaft and connected to the pairs of potentiometers to switch simultaneously from one potentiometer of each pair to the other every 180* of input shaft rotation. 